1. Field of the Invention
The present invention relates to a fluorescent fiber type light detecting device which is suitable for detecting a luminescence condition and a luminescence position for a weak light.
2. Description of the Related Art
In the prior art, a gas insulated switchgear employed, for example, in a substation facility and so forth, has a construction, in which a gas, such as sulfur hexafluoride (SF.sub.6) gas is enclosed within a metallic casing and a plurality of switches and interrupters are arranged in the sulfur hexafluoride (SF.sub.6) atmosphere.
Such gas insulated switchgear is adapted to perform switching at substantially high current. Therefore, an arc discharge can be generated between the switch and casing due to fatigue of components or the sulfur hexafluoride (SF.sub.6) gas. Also, a weak corona discharge resulting from a local high electric field can be caused due to presence of a shape projection or a floating metallic impurity within the casing. If such a corona discharge is left without any treatment, the insulation performance of the equipment is lowered to possibly cause a failure resulting in earth-fault, short-circuit or so forth. In the prior art, in order to monitor such corona discharge, sensors for electrically detecting a voltage, a current, or an acoustic vibration generated in association with the discharge have been used. However, due to the presence of external noise, such as electromagnetic noise, vibration and so forth, an improvement for reliability of the detecting performance of such sensors has been desired.
Hereinafter, discussion will be presented of a conventional method for detecting a luminous condition due to corona discharge and so forth, with reference to FIGS. 18 to 22.
FIG. 8 is a section of a luminescence position detecting device employing a plurality of fluorescent fibers, in which a plurality of optical fiber sensors 1a, 1b and 1c are arranged in parallel. Each of the optical fiber sensors 1a, 1b and 1c is provided with a fluorescent fiber F, in which a fluorescent coloring matter, such as BBOT (2,5-bis[5-tert-2butybenzox azoyl]thiophene), is doped, at one end of a transparent optical fiber 2, and a light receiving element 3 is provided at the other end.
The fluorescent fibers F of respective optical fiber sensors 1a, 1b, 1c are arranged in mutually offset positions in the longitudinal direction of the optical fiber sensors. The side surfaces of the fluorescent fibers F are directed to opposing positions where luminescing in the SF.sub.6 (sulfur hexafluoride) gas is expected.
Since the fluorescent fiber doped with fluorescent coloring matter including BBOT, i.e. the fluorescent fiber doped with BBOT or mixture of BBOT and other fluorescent coloring matter, reacts to luminescing in the SF.sub.6 gas, it can be used for detecting the luminescence position.
In addition, the fluorescent fiber F fluoresces even in response to the light inciding from its side surface, so luminescing in a wide area can be detected by arranging the fluorescent fiber to direct the side surface toward the position where luminescing is expected. Namely, by expanding the lengths L of the portions of the fluorescent fibers in the optical fiber sensors 1a, 1b and 1c of FIG. 18, the area for detecting the luminescing can be expanded.
Assuming that a discharge luminescing is caused for some reason in the SF.sub.6 gas at a position opposing the side surface of the fluorescent fiber F of the intermediate optical fiber sensor 1b, the generated light incides through the side surface of the fluorescent fiber F. By the incident light, the fluorescent material in the fluorescent fiber F is excited to generate fluorescence 5. The fluorescence propagates through the transparent fiber 2 and is detected by the light receiving element 3. As a result, the fact that the light receiving element of the optical fiber sensor 1b receives the light, can be detected by detecting the output signal of the light receiving element 3. Also, by this, it can be determined that the luminescence position is the position opposing the fluorescent fiber F of the intermediate optical fiber sensor 1b.
As set forth above, by arranging the fluorescent fibers of the optical fiber sensors with an offset in a magnitude corresponding to the length L of the fluorescent fiber, luminescing in the area of the 3L length can be detected with three optical fiber sensors. On the other hand, by setting the length L of each fluorescent fiber shorter and setting the magnitude of offsetting of the fluorescent fibers smaller, higher resolution can be achieved thus enabling detection of the luminescing position with high precision, though the number of required optical fiber sensors is increased.
FIG. 19 shows a section of a luminescence position detecting device employing a single fluorescent fiber. The single optical fiber sensor 7 is arranged to direct the side surface thereof to the position where luminescing in the SF.sub.6 gas is expected. This optical fiber sensor is constructed by arranging the light receiving elements 3a and 3b at both ends of the fluorescent fiber F which is formed by doping with a fluorescent coloring matter including BBOT. Both of the light receiving elements 3a and 3b are connected to a comparator circuit 6 so that the luminescence position is detected by comparing the output values of both light receiving elements 3a and 3b.
In this device, if luminescing 4 is caused due to discharge in the SF.sub.6 gas, the fluorescent material at the position corresponding to the position of the luminescing 4 fluoresces. Since the incident position of the discharge luminescence 4 on the side surface of the fluorescent fiber is a shorter distance L2 away from the right side light receiving element 3b than that of L1 from the left side light receiving element 3a, a transmission loss to reach the left side light receiving element 3a is greater than that to reach the right side light receiving element 3b. As a result, by comparing the light receiving magnitudes of the light receiving elements 3a and 3b in the comparator 6, the position of the discharge luminescing can be detected.
FIG. 20 is an explanatory illustration for explaining fluorescent converting action in the fluorescent fiber formed by doping BBOT on a polycarbonate. The reference numeral 8 denotes a core of the fluorescent fiber, which is formed by doping BBOT to polycarbonate. The outer periphery of the core 8 is covered with a clad 9. The external diameter of the clad is on an order of 1 mm.phi..
Assuming that a refraction index of the core is n.sub.1 and a refraction index of the clad 9 is n.sub.2, it is established that n.sub.1 &gt;n.sub.2. On the other hand, assuming a wavelength of the incident light 10 through the side surface of the fluorescent fiber is .lambda..sub.1, and a wavelength of fluorescence 11 propagating with total reflection in the core 8 is .lambda..sub.2, a relationship of .lambda..sub.2 &lt;.lambda..sub.2 is established.
As the core 8, polycarbonate doped with 0.02 Wt % of BBOT is used. On the other hand, as the clad 9, a mixture of polymethyl methacrylate and polyvinylidene fluoride is used. Considering that the particles of the BBOT fluorescent material doped in the polycarbonate are represented by the reference numeral 12, when the incident light 10 having the wavelength .lambda..sub.1 is irradiated from the side surface of the fluorescent fiber on the BBOT fluorescent material particles 12, the BBOT fluorescent material particles 12 fluoresce with the fluorescence having a wavelength .lambda..sub.2 which is longer than .lambda..sub.1.
The luminescence propagates in all directions. However, when it incides in the clad 9, the component having a greater incident angle than a critical angle .theta..sub.c propagates to the end of the core repeating a full reflection within the core. The component having a smaller incident angle than the critical angle .theta..sub.c escapes to the outside passing through the core 8 and the clad 9.
Accordingly, it becomes necessary for detecting weak luminescing in the SF.sub.6 gas so that, when the incident light 10 incides from the side surface of the fluorescent fiber, BBOT fluorescent material particles 12 effectively fluoresce, and the large amount of thus generated fluorescence propagates as full reflected light 11 to exit through the end.
FIG. 21 shows a section illustrating an example implementing the fluorescent fiber type luminescence position detecting device as shown in FIGS. 18 and 19 in a gas insulated switchgear. The gas insulated switchgear employed in a substation facility and so forth has a construction in which the SF.sub.6 gas is enclosed within a metallic tank and a plurality of switches 13 . . . are arranged within the SF.sub.6 gas atmosphere.
Since each switch 13 . . . performs switching at an extremely high voltage current, an arc discharge 23 can be caused due to fatigue of the components of the device or fatigue of the SF.sub.6 gas, and, alternatively, local luminescence 25 can be induced by foreign matter 24 when the foreign matter is present in the SF.sub.6 gas atmosphere. When such discharge is caused, it becomes necessary to detect this to effect treatment, such as maintenance.
In FIG. 21, a denotes a luminescence position detecting device employing a plurality of optical fiber sensors as shown in FIG. 18, and b denotes a luminescence position detecting device employing a single optical fiber as shown in FIG. 19.
The luminescence position detecting devices a and b are disposed within the tank 14 which encloses SF.sub.6 gas and houses the voltage switches 13 . . . . In the tank 14, the luminescence position detecting device a includes a plurality of optical fiber sensors 1a, 1b . . . which are shown in FIG. 18, within a pipe 15 and houses respective light receiving elements within an output opening 16, from which an output signal line 17 is extended. n optical fibers are employed so that the fluorescent fiber portions thereof are arranged offset at a given pitch L so as not to leave a position where the fluorescent fiber portion is not present, over a detecting area nL.
Therefore, in the area nL, when a discharge is caused, the discharge light incides though the side surface of the fluorescent fiber of the optical fiber sensor and is converted into a fluorescent light. The light receiving element converts the fluorescent light into an electrical signal to be output through the output signal line 17. Then, the luminescence position can be detected depending upon the output signal line, on which the output signal occurs.
Within the tank 14, the luminescence position detecting device b has a single optical fiber sensor 7 as shown in FIG. 19. The light receiving elements at both ends are housed at the output openings 19 and 20, from which output signal lines 21 and 22 are extended. The output signal lines 21 and 22 are connected to the comparator 6. Therefore, by comparing the detection signals of the light receiving elements at both ends of the optical fiber sensor 7, the luminescence position of the discharge light in a detecting area L can be detected.
In order to shorten a period required for recovery when failure is caused in the gas insulated switchgear and thus stably supply electric power, it is required to provide a preliminary diagnosis system for detecting failure. Since the gas insulated switchgear causes a weak local discharge from a faulty portion as a precursor to failure, it has been proposed to employ a light detecting device employing the fluorescent fiber for effectively detecting such weak light.
FIG. 22 is a section taken along line B--B of FIG. 21. The reference numeral 4 denotes a luminescence due to an arc discharge or local luminescence. Such luminescence is not always caused at the side of the optical fiber sensor 7. If luminescing is caused at the opposite side of the optical fiber sensor, it is possible that the optical fiber sensor 7 cannot receive the light due to the light being blocked by the high voltage switches 13 . . . .
On the other hand, since the optical fiber sensor 7 is formed in the form of a thin line to provide a small light receiving area, only a small amount of light can be caught by the fluorescent fiber of the optical fiber even when the luminescing is caused at the side of the optical fiber sensor. Therefore, detection of the light cannot be satisfactorily reliable. Furthermore, the light intensity inciding in the optical fiber sensor is inversely proportional to the square of the distance between the luminescence position and the optical fiber sensor 7, so it is possible that the luminescence cannot be detected accurately when the luminescence is caused at a position far away from the optical fiber sensor.